The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. These abstract operations are not a part of the language; they are defined here to aid the specification of the semantics of the language. The conversion abstract operations are polymorphic; that is, they can accept a value of any ECMAScript language type, but not of specification types.
The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The abstract operation ToPrimitive converts its input argument to a nonObject type. If an object is capable of converting to more than one primitive type, it may use the optional hint PreferredType to favour that type. Conversion occurs according to Table 10:
Input Type  Result 

Undefined  The result equals the input argument (no conversion). 
Null  The result equals the input argument (no conversion). 
Boolean  The result equals the input argument (no conversion). 
Number  The result equals the input argument (no conversion). 
String  The result equals the input argument (no conversion). 
Object  Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8. 
The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:
Argument Type  Result 

Undefined  false 
Null  false 
Boolean  The result equals the input argument (no conversion). 
Number  The result is false if the argument is +0, −0, or NaN; otherwise the result is true. 
String  The result is false if the argument is the empty String (its length is zero); otherwise the result is true. 
Object  true 
The abstract operation ToNumber converts its argument to a value of type Number according to Table 12:
Argument Type  Result 

Undefined  NaN 
Null  +0 
Boolean  The result is 1 if the argument is true. The result is +0 if the argument is false. 
Number  The result equals the input argument (no conversion). 
String  See grammar and note below. 
Object 
Apply the following steps:

ToNumber applied to Strings applies the following grammar to the input String. If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.
+
StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral.
DecimalDigits_{opt} ExponentPart_{opt}.
DecimalDigits ExponentPart_{opt}0
1
2
3
4
5
6
7
8
9
e
E
+
DecimalDigits
DecimalDigits0x
HexDigit0X
HexDigit0
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
A
B
C
D
E
F
Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see 7.8.3):
A StringNumericLiteral may be preceded and/or followed by white space and/or line terminators.
A StringNumericLiteral that is decimal may have any number of leading 0
digits.
A StringNumericLiteral that is decimal may be preceded by +
or 
to
indicate its sign.
A StringNumericLiteral that is empty or contains only white space is converted to +0.
The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 7.8.3), but some of the details are different, so the process for converting a String numeric literal to a value of Number type is given here in full. This value is determined in two steps: first, a mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described below.
The MV of StringNumericLiteral ::: [empty] is 0.
The MV of StringNumericLiteral ::: StrWhiteSpace is 0.
The MV of StringNumericLiteral ::: StrWhiteSpace_{opt} StrNumericLiteral StrWhiteSpace_{opt} is the MV of StrNumericLiteral, no matter whether white space is present or not.
The MV of StrNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.
The MV of StrNumericLiteral ::: HexIntegerLiteral is the MV of HexIntegerLiteral.
The MV of StrDecimalLiteral ::: StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
The MV of StrDecimalLiteral ::: +
StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
The MV of StrDecimalLiteral ::: 
StrUnsignedDecimalLiteral is the negative of the MV of StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral
is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless
mathematical zero to a floatingpoint +0 or −0 as appropriate.)
The MV of StrUnsignedDecimalLiteral ::: Infinity is 10^{10000} (a value so large that it will round to +∞).
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits .
is the MV of DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits .
DecimalDigits is the MV of the
first DecimalDigits plus (the MV of the second DecimalDigits times 10^{−n}), where n is the number of
characters in the second DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits .
ExponentPart is the MV of
DecimalDigits times 10^{e}, where e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits .
DecimalDigits ExponentPart is (the MV of the first DecimalDigits plus (the MV of the second
DecimalDigits times 10^{−n})) times 10^{e}, where n is the number of
characters in the second DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral ::: .
DecimalDigits is the MV of DecimalDigits times
10^{−n}, where n is the number of characters in DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: .
DecimalDigits ExponentPart is the MV of
DecimalDigits times 10^{e−n}, where n is the number of characters in
DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits is the MV of DecimalDigits.
The MV of StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPart is the MV of DecimalDigits times 10^{e}, where e is the MV of ExponentPart.
The MV of DecimalDigits ::: DecimalDigit is the MV of DecimalDigit.
The MV of DecimalDigits ::: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.
The MV of ExponentPart ::: ExponentIndicator SignedInteger is the MV of SignedInteger.
The MV of SignedInteger ::: DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger ::: +
DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger ::: 
DecimalDigits is the negative of the MV of DecimalDigits.
The MV of DecimalDigit ::: 0
or of HexDigit ::: 0
is 0.
The MV of DecimalDigit ::: 1
or of HexDigit ::: 1
is 1.
The MV of DecimalDigit ::: 2
or of HexDigit ::: 2
is 2.
The MV of DecimalDigit ::: 3
or of HexDigit ::: 3
is 3.
The MV of DecimalDigit ::: 4
or of HexDigit ::: 4
is 4.
The MV of DecimalDigit ::: 5
or of HexDigit ::: 5
is 5.
The MV of DecimalDigit ::: 6
or of HexDigit ::: 6
is 6.
The MV of DecimalDigit ::: 7
or of HexDigit ::: 7
is 7.
The MV of DecimalDigit ::: 8
or of HexDigit ::: 8
is 8.
The MV of DecimalDigit ::: 9
or of HexDigit ::: 9
is 9.
The MV of HexDigit ::: a
or of HexDigit ::: A
is 10.
The MV of HexDigit ::: b
or of HexDigit ::: B
is 11.
The MV of HexDigit ::: c
or of HexDigit ::: C
is 12.
The MV of HexDigit ::: d
or of HexDigit ::: D
is 13.
The MV of HexDigit ::: e
or of HexDigit ::: E
is 14.
The MV of HexDigit ::: f
or of HexDigit ::: F
is 15.
The MV of HexIntegerLiteral ::: 0x
HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral ::: 0X
HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral ::: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.
Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If
the MV is 0, then the rounded value is +0 unless the first non white space character in the String numeric literal is
‘
’, in which case the rounded value is −0. Otherwise, the rounded value must be the Number
value for the MV (in the sense defined in 8.5), unless the literal includes a StrUnsignedDecimalLiteral
and the literal has more than 20 significant digits, in which case the
Number
value may be either the Number value for the MV of a literal
produced by replacing each significant digit after the 20th
with a 0 digit or the Number value for the MV of a literal
produced by replacing each significant digit after the 20th with
a 0 digit and then incrementing the literal at the 20th digit
position. A digit is significant if it is not part of
an ExponentPart and
The abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation functions as follows:
The abstract operation ToInt32 converts its argument to one of 2^{32} integer values in the range −2^{31} through 2^{31}−1, inclusive. This abstract operation functions as follows:
NOTE Given the above definition of ToInt32:
The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)
ToInt32 maps −0 to +0.
The abstract operation ToUint32 converts its argument to one of 2^{32} integer values in the range 0 through 2^{32}−1, inclusive. This abstraction operation functions as follows:
NOTE Given the above definition of ToUInt32:
Step 5 is the only difference between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)
ToUint32 maps −0 to +0.
The abstract operation ToUint16 converts its argument to one of 2^{16} integer values in the range 0 through 2^{16}−1, inclusive. This abstract operation functions as follows:
NOTE Given the above definition of ToUint16:
The abstract operation ToString converts its argument to a value of type String according to Table 13:
Argument Type  Result 

Undefined  "undefined" 
Null  "null" 
Boolean 
If the argument is true, then the result is If the argument is false, then the result is "false". 
Number  See 9.8.1. 
String  Return the input argument (no conversion) 
Object 
Apply the following steps: 1. Let primValue be ToPrimitive(input argument, hint String). 2. Return ToString(primValue). 
The abstract operation ToString converts a Number m to String format as follows:
"NaN"
."0"
.""
and ToString(−m)."Infinity"
.0
’..
’, followed by the remaining
k−n digits of the decimal representation of s.0
’, followed by a
decimal point ‘.
’, followed by −n occurrences of the character
‘0
’, followed by the k digits of the decimal representation of s.e
’, followed by a plus sign ‘+
’ or minus sign
‘−
’ according to whether n−1 is positive or negative, followed by the
decimal representation of the integer abs(n−1) (with no leading zeroes).e
’, followed by a plus sign
‘+
’ or minus sign ‘−
’ according to whether n−1 is
positive or negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeroes).NOTE 1 The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:
NOTE 2 For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:
Otherwise, let n, k, and s be integers such that k ≥ 1, 10^{k−1} ≤ s < 10^{k}, the Number value for s × 10^{n−k} is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10^{n−k} is closest in value to m. If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.
NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binarytodecimal conversion of floatingpoint numbers:
Gay, David M. Correctly Rounded BinaryDecimal and DecimalBinary Conversions. Numerical Analysis, Manuscript 9010.
AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available as
http://cm.belllabs.com/cm/cs/doc/90/410.ps.gz. Associated code
available as
http://cm.belllabs.com/netlib/fp/dtoa.c.gz and as
http://cm.belllabs.com/netlib/fp/g_fmt.c.gz and may also be found
at the various netlib
mirror sites.
The abstract operation ToObject converts its argument to a value of type Object according to Table 14:
Argument Type  Result 

Undefined  Throw a TypeError exception. 
Null  Throw a TypeError exception. 
Boolean  Create a new Boolean object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.6 for a description of Boolean objects. 
Number  Create a new Number object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.7 for a description of Number objects. 
String  Create a new String object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.5 for a description of String objects. 
Object  The result is the input argument (no conversion). 
The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 15:
Argument Type  Result 

Undefined  Throw a TypeError exception. 
Null  Throw a TypeError exception. 
Boolean  Return 
Number  Return 
String  Return 
Object  Return 
The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value, is a callable function Object according to Table 16:
Argument Type  Result 

Undefined  Return false. 
Null  Return false. 
Boolean  Return false. 
Number  Return false. 
String  Return false. 
Object  If the argument object has a [[Call]] internal method, then return true, otherwise return false. 
The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows: