Function: getTimeToFirstByte()

function getTimeToFirstByte(sample): number;

Defined in: getTimeToFirstByte.ts:18

Returns the time to first byte of a response, in milliseconds.

When responseStart is a finite number between startTime and the end of the response, the function returns the time from startTime to responseStart. Otherwise the function returns NaN. The time to first byte and the transfer from getTransferDuration add up to duration.

Parameters

sample

ResourceTiming

A ResourceTiming sample

Returns

number

The time to first byte in milliseconds, or NaN when the sample has no usable responseStart

Example

// 800 ms to the first byte, then a 50 ms transfer.
const timeToFirstByteMs = getTimeToFirstByte({
	startTime: 1000,
	responseStart: 1800,
	duration: 850,
	encodedBodySize: 112_000,
})

equal(timeToFirstByteMs, 800)