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69 changes: 69 additions & 0 deletions
69
files/en-us/web/javascript/reference/errors/bad_await/index.md
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,69 @@ | ||
| --- | ||
| title: "SyntaxError: await is only valid in async functions, async generators and modules" | ||
| slug: Web/JavaScript/Reference/Errors/Bad_await | ||
| page-type: javascript-error | ||
| --- | ||
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| {{jsSidebar("Errors")}} | ||
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| The JavaScript exception "await is only valid in async functions, async generators and modules" occurs when an {{jsxref("Operators/await", "await")}} expression is used outside of [async functions](/en-US/docs/Web/JavaScript/Reference/Statements/async_function) or [modules](/en-US/docs/Web/JavaScript/Guide/Modules) or other async contexts. | ||
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| ## Message | ||
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| ``` | ||
| SyntaxError: await is only valid in async functions and the top level bodies of modules (V8-based) | ||
| SyntaxError: await is only valid in async functions, async generators and modules (Firefox) | ||
| SyntaxError: Unexpected identifier (Safari) | ||
| ``` | ||
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| ## Error type | ||
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| {{jsxref("SyntaxError")}}. | ||
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| ## What went wrong? | ||
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| JavaScript execution is never blocking: an `await` can never block the execution of the program. Instead, it pauses the execution of the surrounding async task, while allowing other tasks to continue running. Therefore, `await` cannot be used in sync tasks, such as functions, generator functions, or top level of scripts. It is not always apparent whether the current file is a script or a module — see the [Modules guide](/en-US/docs/Web/JavaScript/Guide/Modules#top_level_await) for more information. | ||
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| ## Examples | ||
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| ### Top-level await | ||
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| You cannot use `await` at the top level of a script: | ||
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| ```html example-bad | ||
| <script> | ||
| await fetch("https://example.com"); | ||
| // SyntaxError: await is only valid in async functions, async generators and modules | ||
| </script> | ||
| ``` | ||
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| Instead, make the script a module: | ||
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| ```html example-good | ||
| <script type="module"> | ||
| await fetch("https://example.com"); | ||
| </script> | ||
| ``` | ||
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| ### Async callbacks | ||
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| You cannot use `await` in a sync callback: | ||
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| ```js example-bad | ||
| urls.forEach((url) => { | ||
| await fetch(url); | ||
| // SyntaxError: await is only valid in async functions, async generators and modules | ||
| }); | ||
| ``` | ||
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| Instead, make the callback async. See more explanation in the [Using promises guide](/en-US/docs/Web/JavaScript/Guide/Using_promises#composition). | ||
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| ```js example-good | ||
| Promise.all(urls.map(async (url) => { | ||
| await fetch(url); | ||
| }); | ||
| ``` | ||
| ## See also | ||
| - {{jsxref("Operators/await", "await")}} |
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files/en-us/web/javascript/reference/errors/bad_break/index.md
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,134 @@ | ||
| --- | ||
| title: "SyntaxError: unlabeled break must be inside loop or switch" | ||
| slug: Web/JavaScript/Reference/Errors/Bad_break | ||
| page-type: javascript-error | ||
| --- | ||
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| {{jsSidebar("Errors")}} | ||
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| The JavaScript exception "unlabeled break must be inside loop or switch" occurs when a {{jsxref("Statements/break", "break")}} statement is not inside a loop or a {{jsxref("Statements/switch", "switch")}} statement. | ||
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| ## Message | ||
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| ``` | ||
| SyntaxError: Illegal break statement (V8-based) | ||
| SyntaxError: unlabeled break must be inside loop or switch (Firefox) | ||
| SyntaxError: 'break' is only valid inside a switch or loop statement. (Safari) | ||
| ``` | ||
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| ## Error type | ||
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| {{jsxref("SyntaxError")}}. | ||
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| ## What went wrong? | ||
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| {{jsxref("Statements/break", "break")}} statements can be used to exit a loop or a `switch` statement, and using them elsewhere is a syntax error. Alternatively, you can provide a [label](/en-US/docs/Web/JavaScript/Reference/Statements/label) to the `break` statement to break out of any statement with that label — however, if the label does not reference a containing statement, another error [SyntaxError: label not found](/en-US/docs/Web/JavaScript/Reference/Errors/Label_not_found) will be thrown. | ||
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| ## Examples | ||
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| ### Unsyntactic break | ||
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| `break` cannot be used outside `switch` or loops. | ||
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| ```js example-bad | ||
| let score = 0; | ||
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| function increment() { | ||
| if (score === 100) | ||
| break; // SyntaxError: unlabeled break must be inside loop or switch | ||
| } | ||
| score++; | ||
| } | ||
| ``` | ||
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| Maybe instead of `break`, you intend to use {{jsxref("Statements/return", "return")}} to early-terminate a function. | ||
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| ```js example-good | ||
| let score = 0; | ||
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| function increment() { | ||
| if (score === 100) | ||
| return; | ||
| } | ||
| score++; | ||
| } | ||
| ``` | ||
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| ### Using break in callbacks | ||
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| `break` cannot be used in callbacks, even if the callback is called from a loop. | ||
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| ```js example-bad | ||
| let containingIndex = 0; | ||
| const matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]; | ||
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| while (containingIndex < matrix.length) { | ||
| matrix[containingIndex].forEach((value) => { | ||
| if (value === 5) { | ||
| break; // SyntaxError: unlabeled break must be inside loop or switch | ||
| } | ||
| }); | ||
| containingIndex++; | ||
| } | ||
| ``` | ||
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| Instead, refactor the code so the `break` is used outside the callback. | ||
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| ```js example-good | ||
| let containingIndex = 0; | ||
| const matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]; | ||
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| outer: while (containingIndex < matrix.length) { | ||
| for (const value of matrix[containingIndex]) { | ||
| if (value === 5) { | ||
| break outer; | ||
| } | ||
| } | ||
| containingIndex++; | ||
| } | ||
| ``` | ||
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| ```js example-good | ||
| let containingIndex = 0; | ||
| const matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]; | ||
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| while (containingIndex < matrix.length) { | ||
| if (matrix[containingIndex].includes(5)) | ||
| break; | ||
| } | ||
| containingIndex++; | ||
| } | ||
| ``` | ||
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| There's no way to early-terminate a {{jsxref("Array/forEach", "forEach()")}} loop. You can use {{jsxref("Array/some", "some()")}} instead, or convert it to a {{jsxref("Statements/for...of", "for...of")}} loop. | ||
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| ```js example-bad | ||
| array.forEach((value) => { | ||
| if (value === 5) { | ||
| break; // SyntaxError: unlabeled break must be inside loop or switch | ||
| } | ||
| // do something with value | ||
| }); | ||
| ``` | ||
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| ```js example-good | ||
| array.some((value) => { | ||
| if (value === 5) { | ||
| return true; | ||
| } | ||
| // do something with value | ||
| }); | ||
| ``` | ||
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| ```js example-good | ||
| for (const value of array) { | ||
| if (value === 5) { | ||
| break; | ||
| } | ||
| // do something with value | ||
| } | ||
| ``` | ||
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| ## See also | ||
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| - {{jsxref("Statements/break", "break")}} |
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