operator-assignment.md

require or disallow assignment operator shorthand where possible (operator-assignment)

JavaScript provides shorthand operators that combine variable assignment and some simple mathematical operations. For example, x = x + 4 can be shortened to x += 4. The supported shorthand forms are as follows:

 Shorthand | Separate
-----------|------------
 x += y    | x = x + y
 x -= y    | x = x - y
 x *= y    | x = x * y
 x /= y    | x = x / y
 x %= y    | x = x % y
 x **= y   | x = x ** y
 x <<= y   | x = x << y
 x >>= y   | x = x >> y
 x >>>= y  | x = x >>> y
 x &= y    | x = x & y
 x ^= y    | x = x ^ y
 x |= y    | x = x | y

Rule Details

This rule requires or disallows assignment operator shorthand where possible.

Options

This rule has a single string option:

• "always" (default) requires assignment operator shorthand where possible
• "never" disallows assignment operator shorthand

always

Examples of incorrect code for this rule with the default "always" option:

/*eslint operator-assignment: ["error", "always"]*/

x = x + y;
x = y * x;
x[0] = x[0] / y;
x.y = x.y << z;

Examples of correct code for this rule with the default "always" option:

/*eslint operator-assignment: ["error", "always"]*/

x = y;
x += y;
x = y * z;
x = (x * y) * z;
x[0] /= y;
x[foo()] = x[foo()] % 2;
x = y + x; // `+` is not always commutative (e.g. x = "abc")

never

Examples of incorrect code for this rule with the "never" option:

/*eslint operator-assignment: ["error", "never"]*/

x *= y;
x ^= (y + z) / foo();

Examples of correct code for this rule with the "never" option:

/*eslint operator-assignment: ["error", "never"]*/

x = x + y;
x.y = x.y / a.b;

When Not To Use It

Use of operator assignment shorthand is a stylistic choice. Leaving this rule turned off would allow developers to choose which style is more readable on a case-by-case basis.