fix(gatsby-remark-autolink-headers): remove hardcoded "let" keyword

[ivan-aksamentov]

Description

In gatsby-remark-autolink-headers, this code snippet

gatsby/packages/gatsby-remark-autolink-headers/src/gatsby-ssr.js

Lines 51 to 67 in 5e7ccd1

	const script = `
	document.addEventListener("DOMContentLoaded", function(event) {
	var hash = window.decodeURI(location.hash.replace('#', ''))
	if (hash !== '') {
	var element = document.getElementById(hash)
	if (element) {
	let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
	let clientTop = document.documentElement.clientTop || document.body.clientTop || 0
	var offset = element.getBoundingClientRect().top + scrollTop - clientTop
	// Wait for the browser to finish rendering before scrolling.
	setTimeout((function() {
	window.scrollTo(0, offset - ${offsetY})
	}), 0)
	}
	}
	})
	`

contains 2 instances of hardcoded "let" keyword, which is being rendered into the HTML without transpilation. This makes the script crash in old browsers, such as IE (all versions).

This commit replaces "let" to "var". Care should be taken because this is an unsafe transformation, due to var scope hoisting. My initial tests show that everything is good.

I initially encountered this error while porting https://nextstrain.org to IE: nextstrain/nextstrain.org#113

This fix should help scientists researching 2019-nCoV outbreak (some of them still use Internet Explorer)

Documentation

N/A

Related Issues

Fixes #21058

[pieh]

Ok, this makes sense - can we add comment above

  const script = `
    document.addEventListener("DOMContentLoaded", function(event) {	    document.addEventListener("DOMContentLoaded", function(event) {

part to mention that using var is intentional (and why we can't use let)

[ivan-aksamentov]

@pieh Hi Michal, thanks for the quick response!
I added a comment explaining the change.

[pieh]

Looks good! Thanks @ivan-aksamentov!

[pieh]

Yup, we already used var there (but not consistently). The comment is mostly as a measure to avoid regressions in the future if someone tries to change it to let again.

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