Why is my JavaScript not working correctly in strict mode on Safari?

Question

I have built a website that uses some simple JavaScript. After some testing, I have found that my JavaScript is behaving very differently on iOS devices as compared to all other devices that I could test with.

After several hours of trial-and-error, I have discovered that the unexpected behaviour occurs only in strict mode, but it has been very hard to troubleshoot the problem further because I am limited in my software/hardware for Apple development and testing.

Why could my code not be working in strict mode, and only certain (Apple, in particular) devices?

Answer 1

The possible problem

Your problem may be that you have made a const declaration with strict mode enabled.

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A possible solution

If that is the case, and you would like to continue using strict mode, one solution is to use var instead of const , and simply be more careful to treat the variable as a constant.

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More on this problem

It is in fact difficult to debug this problem without an OS X computer, because you are unable to use iOS Safari's remote console "Web Inspector".

However, according to caniuse , for the current versions of both Safari and iOS Safari (9.1 and 9.3, respectively, as of writing) const is:

Only recognized when NOT in strict mode

You can test this for yourself using the following example ( or this JSFiddle ). This code worked as expected for me with Chrome and Firefox on both Ubuntu and Android, but not on Firefox for iOS or iOS Safari.

HTML

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<pre></pre>
<form>
    <input type="submit">
</form>

JS

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"use strict";

var form = document.querySelector("form");
var info = document.querySelector("pre");

// This const declaration in conjunction with strict mode will
// cause this script not to work on iOS devices
const _MEANINGLESS = 0;

form.addEventListener("submit", function(event) {
    event.preventDefault();
    info.textContent = "Submit event captured.\n";
});