requireHtmlOutput

function requireHtmlOutput(
  value: unknown,
  message: string = "Expected an HtmlOutput object."
): HtmlOutput;

An Apps Script service object has no constructor a script can test against, so the guard looks for the methods that define the type — getContent, setTitle, setXFrameOptionsMode. A stub that implements them passes, which is what makes code using this guard testable outside the runtime.

isX and nonX answer a question and never interrupt control flow; requireX and requireNonX hand the value back or throw. The scheme is explained in Validation conventions.

Parameters

Parameter	Type	Description
`value`	`unknown`	The value to validate.
`message` (optional) `= "Expected an HtmlOutput object."`	`string`	The message of the exception. A default is used when omitted.

Returns

HtmlOutput — the same value, typed as an HtmlOutput.

Throws

Exception	Condition
`IllegalArgumentException`	the value is not an HtmlOutput.

Examples

What passes and what does not

const output = HtmlService.createHtmlOutput("<p>hi</p>");

requireHtmlOutput(output); // => output
requireHtmlOutput({}); // throws IllegalArgumentException
requireHtmlOutput(null); // throws IllegalArgumentException

Source

src/appsscript/ui/requireHtmlOutput.ts

23 September 2026

This documentation was generated with AI (Claude) from the library's source code.

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