JS: Assignment Operators

By Xah Lee. Date: . Last updated: .

Assignment Operators

var = val
assign val to var, and return val [see let Declaration] 
let zz;
console.log(((zz = 3) + 1) === 4);

Increment / Decrement Assignment

++x
same as x=x+1 (return new value)
--x
same as x=x-1 (return new value)
x++
like x=x+1 but return old value
x--
like x=x-1 but return old value

Compound Assignment Operators

x += y
same as x = x + y
x -= y
same as x = x - y
x *= y
same as x = x * y
x /= y
same as x = x / y
x %= y
same as x = x % y

Logical Assignment Operators

x ??= y
(JS2021) LOGICAL Nullish assignment operator.
Assigns y to x if x is null or undefined .

left-hand-side must be assignable, such as a variable or property.

typically used for assigning a value to a object property but only when the property does not exist. [see Get Set Property] 

let x;
// if x has no value (null or undefined), give it one
x ??= 4;
console.log(x === 4);
let x = false;
// if x has no value (null or undefined), give it one
x ??= 4;
console.log((x === 4) === false);
// assignment failed, because x do have a value already
x ||= y
(JS2021) LOGICAL OR assignment operator.
Assigns y to x if Boolean(x) eval to false. [see JS: Boolean: true, false]

left-hand-side must be assignable, such as a variable or property.

typically used for assigning a value to a object property but only when the property does not exist or has no β€œvalid” value (e.g. false, null or undefined). [see Get Set Property] 

let x;
// if x has no value (null or undefined), give it one
x ||= 4;
console.log(x === 4);
x &&= y
(JS2021) LOGICAL AND assignment operator.
Assigns y to x if Boolean(x) eval to true. [see JS: Boolean: true, false]

left-hand-side must be assignable, such as a variable or property. 

let x = 3;
x &&= 4;
// assignment successful, because x eval to true
console.log(x === 4);
let x;
x &&= 4;
// assignment fails because x was false
console.log(x === undefined);

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