Typescript throwing error during unit tests that variable undefined even though I have declare var statement at beginning of file

Question

I'm using angular(2) and typescript. I'm using the moment library to convert dates so I have this function inside one of my utility classes:

static isoStringToDateObj(isoDateString: string): Date {
    // For unit tests and any environment where moment library is not present
    if (!moment) {
        return new Date(isoDateString);
    }
    return moment(isoDateString).toDate();
}

This works fine in the browser, but when I try to run my unit tests it throws the error: ReferenceError: Can't find variable: moment in karma-test-shim.js (line 16240)

The line it is referring to is the if (!moment) line.

Why does it care if the variable does not exist in a statement that is only checking if the variable exists? It is not referencing any properties on it.
I have declare var moment: any; at the top of the file that the function isoStringToDateObj is in, so why is it saying reference error when the variable is actually declared?

score 2 · Accepted Answer · answered Jul 05 '17 at 21:17

2

The only proper way to check if global variable does not exist is:

if (typeof moment === 'undefined') { ... }

ReferenceError: Can't find variable: moment

is runtime error, declare var moment: any cheats typing system to be silent about that but can't affect actual moment global in any way.

answered Jul 05 '17 at 21:17

Estus Flask

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Note: a much cleaner solution instead of `declare var foo: any` is `declare var foo: foo_type | undefined`, this keeps most of the type-checking while still allowing `undefined`. – biziclop Jul 04 '19 at 11:07

Typescript throwing error during unit tests that variable undefined even though I have declare var statement at beginning of file

1 Answers1