Expected an assignment or function call and instead saw an expression

Question

I'm totally cool with this JSLint error. How can I tolerate it? Is there a flag or checkbox for it?

You get it when you do stuff like:

v && arr.push(v);

as opposed to:

if (v) {
    arr.push(v);
}

Both do the same exact thing. If you put:

window.test = function(v) {
    'use strict';
    var arr = [];
    if (v) {
        arr.push(v);
    }
    return arr;
};

into the minifier it minifies down to this anyway:

window.test=function(a){var b=[];a&&b.push(a);return b};

Solution 1

I don't think JSLint has an option to turn that off.

JSHint (a fork with more options) has an option for it, though: The expr option, documented as "if ExpressionStatement should be allowed as Programs".

Solution 2

You can add the following line to ignore that warning:

/*jshint -W030 */

You can read more about it here.

Solution 3

People who are looking how to suppress it when using ESLint. You can ingnore it by writing the following comment just above the line no-unused-expressions

// eslint-disable-next-line no-unused-expressions

You can also suppress the warning for the entire file by placing the following comment at the very top of the file

/* eslint-disable no-unused-expressions */

Solution 4

There's no option for this in JSLint. You can circumvent it using:

var dummy = v && arr.push(v);

NB: dummy evaluates to true after that.

Another workaround could be:

function expression(statement) { 
 'use strict';
 return statement; 
}
expression(v && arr.push);

View more solutions

Comments

[T.J. Crowder]

@diEcho: You can also download and use it locally (that's what I do). I run it via NodeJS with my preferred options set, dramatically helps my confidence in correct code. :-)

[ryanve]

It'll work but it's silly. The minifier will actually sort that out and but for readability it'd be better to just use the if statement than that IMO.

[KooiInc]

That's true. But you asked for a way to tolerate is. Added another workaround. It's like Crockford said somewhere: jsLint is designed to be a pain in the ass for programmers ;~)

[Arseni Mourzenko]

The answer is completely unrelated to the question, which was about JSLint, not JSHint.

[Charlie Martin]

Yet its still the most useful answer for the majority of people who land here.

[iconoclast]

"completely unrelated"? Because JSHint is completely unrelated to JSLint?

[faebster]

WRONG the wonderful && hack does not work in jsLint... maybe depends on version...

[Mohammad Kermani]

Can describe more please, why that warning happened?

[johannchopin]

I recommend you to also provide the link of the documentation of this specific rule no-unused-expressions