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/libraries/timeToFirstBytesUtils/timeToFirstBytesUtils.js
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/**
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 * Calculates the Time to First Byte (TTFB) for the given window object.
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 *
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 * This function attempts to use the Navigation Timing Level 2 API first, and falls back to
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 * the Navigation Timing Level 1 API if the former is not available.
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 *
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 * @param {Window} win - The window object from which to retrieve performance timing information.
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 * @returns {string} The TTFB in milliseconds as a string, or an empty string if the TTFB cannot be determined.
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 */
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export function getTimeToFirstByte(win) {
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  const performance = win.performance || win.webkitPerformance || win.msPerformance || win.mozPerformance;
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  const ttfbWithTimingV2 = performance &&





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        typeof performance.getEntriesByType === 'function' &&










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        Object.prototype.toString.call(performance.getEntriesByType) === '[object Function]' &&










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        performance.getEntriesByType('navigation')[0] &&










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        performance.getEntriesByType('navigation')[0].responseStart &&










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        performance.getEntriesByType('navigation')[0].requestStart &&










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        performance.getEntriesByType('navigation')[0].responseStart > 0 &&










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        performance.getEntriesByType('navigation')[0].requestStart > 0 &&










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        Math.round(










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          performance.getEntriesByType('navigation')[0].responseStart - performance.getEntriesByType('navigation')[0].requestStart










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        );










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  if (ttfbWithTimingV2) {





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    return ttfbWithTimingV2.toString();




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  }










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  const ttfbWithTimingV1 = performance &&





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        performance.timing.responseStart &&










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        performance.timing.requestStart &&










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        performance.timing.responseStart > 0 &&










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        performance.timing.requestStart > 0 &&










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        performance.timing.responseStart - performance.timing.requestStart;










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  return ttfbWithTimingV1 ? ttfbWithTimingV1.toString() : '';





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}
























    

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