Assignment

Plain assignment

= stores the value on the right in the name on the left. It is not a comparison, which is why comparison needs three characters.

let total = 10;
total = total + 5;

console.log(total);

15

Compound forms

Every arithmetic operator has a shorthand that reads the variable, applies the operation and stores the result.

let n = 10;

n += 5;
n -= 3;
n *= 2;
n /= 4;
n %= 4;

console.log(n);

2

Strings too

+= works on strings, which is how text gets built up a piece at a time.

let message = 'Hello';

message += ', ';
message += 'world';

console.log(message);

Hello, world

Logical assignment

Three newer forms assign only under a condition. They are worth knowing because they replace a very common three-line pattern.

let name = '';
name ||= 'Anonymous';

let count = 0;
count ??= 10;

let flag = true;
flag &&= 'replaced';

console.log(name, count, flag);

Anonymous 0 replaced

||= assigns when the current value is falsy. ??= assigns only when it is null or undefined, which is why count stayed 0 and name did not stay empty. &&= assigns only when the current value is truthy.

Assignment is an expression

An assignment produces the assigned value, which is why chains work. It is also why a mistyped = inside an if is valid code rather than an error.

let a;
let b;

a = b = 5;

console.log(a, b);

5 5