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---
title: Subtraction assignment (-=)
slug: Web/JavaScript/Reference/Operators/Subtraction_assignment
page-type: javascript-operator
browser-compat: javascript.operators.subtraction_assignment
---
{{jsSidebar("Operators")}}
The **subtraction assignment (`-=`)** operator performs [subtraction](/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction) on the two operands and assigns the result to the left operand.
{{EmbedInteractiveExample("pages/js/expressions-subtraction-assignment.html")}}
## Syntax
```js-nolint
x -= y
```
## Description
`x -= y` is equivalent to `x = x - y`, except that the expression `x` is only evaluated once.
## Examples
### Subtraction assignment using numbers
```js
let bar = 5;
bar -= 2; // 3
```
Other non-BigInt values are coerced to numbers:
```js
bar -= "foo"; // NaN
```
### Subtraction assignment using BigInts
```js
let foo = 3n;
foo -= 2n; // 1n
foo -= 1; // TypeError: Cannot mix BigInt and other types, use explicit conversions
```
## Specifications
{{Specifications}}
## Browser compatibility
{{Compat}}
## See also
- [Assignment operators in the JS guide](/en-US/docs/Web/JavaScript/Guide/Expressions_and_operators#assignment_operators)
- [Subtraction (`-`)](/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction)