JS: Logical Assignment Operators

• 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

• 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 “valid” value (e.g. false, null or undefined). 〔see Get Set Property〕

let xx = 0;
// if xx eval to false, give it a new value
xx ||= 4;
console.log(xx === 4);

let yy = 1;
yy ||= 4;
console.log(yy === 1);

• 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);