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javascriptmicrosoft-edgechromium

Why does "x[x.length - 1]" throw "Uncaught SyntaxError: invalid or unexpected token" on an array in the Javascript console?

Edit: a - lookalike character was the culprit.

I have a script I'm pasting into Edge's developer console that breaks with this:

let args;
if (typeof arguments === 'undefined') {
  args = [1, 'January', 2023, 'test', x => console.log(x)];
} else {
  args = arguments;
}
let returnCallback = args[args.length - 1];

The last line is what triggers "Uncaught SyntaxError: invalid or unexpected token".

I can replace args.length - 1 with 4, and the code runs how I expect it to.
I can even replace args.length - 1 with args.length, and no errors are thrown.
I tried pasting all but the last line in, then pressing enter. I confirmed args is defined. Pasting that last line in still throws the error.

This is the minimum code I can reduce it to and still throw the error*:

let x = [1, 2];
x.length - 1;

The following does not throw the error:

let x = [1, 2];
x.length;

When I enter the lines individually, it gets tricky.
First, I will enter let x = [1, 2]; and press enter. If I paste x.length - 1; in and hit enter, I get the error. But if I type x.length - 1;, it will not give the error.
(If I set x to a single-element array, it will still error when pasting the 2 lines together. But pasting them individually always works. It needs 2+ elements for pasting vs typing to matter.)

I don't think it is just chrome cashing x.length when it previews the value as you type it.
For example, look at the result of these 3 lines, all pasted:
let x = [1, 2]; enter
Result: undefined
x.length enter
Result: 2
x.length - 1 enter
Uncaught SyntaxError: Invalid or expected token

The error it gives isn't what you'd expect in that scenario either.
If I run x.length - 1 without declaring x, the output is "Uncaught ReferenceError: x is not defined"
If I run it after let x;, the output is "Uncaught TypeError: Cannot read properties of undefined"
If I set x to something without a length property, the result is NaN

So it's more than the console having an incorrect or no value for the variable.

Is this a problem with my code? Or am I running into a bug on Edge?

Solution

  • Turns out I was using the wrong "-" sign. I was using "−" (U-2212 "Minus Sign") Thanks to Sebastian Simon for suggesting I check for this.