SyntaxError: await/yield expression can't be used in parameter

The JavaScript exception "await expression can't be used in parameter" or "yield expression can't be used in parameter" occurs when the default parameter expression contains the {{jsxref("Operators/await", "await")}} or {{jsxref("Operators/yield", "yield")}} keyword and has the effect of pausing default parameter evaluation.

Message#

SyntaxError: Illegal await-expression in formal parameters of async function (V8-based)
SyntaxError: await expression can't be used in parameter (Firefox)
SyntaxError: Cannot use 'await' within a parameter default expression. (Safari)

SyntaxError: Yield expression not allowed in formal parameter (V8-based)
SyntaxError: yield expression can't be used in parameter (Firefox)
SyntaxError: Unexpected keyword 'yield'. Cannot use yield expression within parameters. (Safari)

Error type#

{{jsxref("SyntaxError")}}

What went wrong?#

The default expression must be able to evaluate synchronously. If it contains an await or yield expression, it will pause the evaluation of the default expression, which is not allowed.

[!NOTE] This error is only generated when await or yield are valid operators in this function context. Otherwise, await or yield would be parsed as an identifier, and either not cause an error, or cause an error like "reserved identifier", or "unexpected token" if there's an expression following it.

Examples#

Invalid cases#

```js example-bad function *gen(a = yield 1) {}

async function f(a = await Promise.resolve(1)) {}

### Valid cases

You can use the [nullish coalescing assignment](/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_assignment) to provide a default value instead. If you want to treat `null` and `undefined` differently, you would need to use a condition.

```js example-good
function* gen(a) {
  a ??= yield 1;
}

async function f(a) {
  a ??= await Promise.resolve(1);
}

You are also allowed to use await or yield if the expression is contained in a function expression of the initializer and would not pause the evaluation of the default expression.

js example-good async function f(a = (async () => await Promise.resolve(1))()) {}

See also#

• Default parameters
• {{jsxref("Operators/await", "await")}}
• {{jsxref("Operators/yield", "yield")}}