JavaScript: Assignment Operators

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 Property, 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 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 “valid” value (e.g. false, null or undefined). [see Get Property, 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 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);