JavaScript anonymous function in the for loop error

Question

Hello I was reading "JavaScript: the definitive guide" 6th edition and tried one of the examples at 9.1 Classes and Prototypes.

function range (from, to) {
    var r = Object.create(range.methods);

    r.from = from;
    r.to = to;
    return r;
}

range.methods = {
    includes: function(x) { 
        return this.from <= x && x <= this.to; 
    },
    foreach: function(f) {
        for(var x = Math.ceil(this.from); x <= this.to; x++) 
            f(x);
    },
    toString: function() { 
        return "(" + this.from + "..." + this.to + ")"; 
    }
};

Loading this into console throws an error

Uncaught TypeError: Illegal invocation class.js:31. range.methods.foreach class.js:31 (anonymous function)

I guess intention of foreach method is to pass a function name as an argument

var r = range(1, 3);
r.foreach(console.log);

Any ideas how to fix this error?

Illegal Invocation error when console.log passed in a function — Brett
– Brett, Commented Jul 29, 2014 at 12:32

dfsq · Accepted Answer · 2014-07-29 12:38:22Z

9

It happens because you detached log method from console object, while console.log expects context (this) to be console, not Window as it becomes when you lose context. If you want to use console.log as a function you should explicitly tell it what context to use.

r.foreach(console.log.bind(console));

Function.prototype.bind is your friend in this case.

For future readers: ES6 style would also allow you to use arrow functions for this which would be even more concise:

r.foreach(x => console.log(x))

edited Jul 29, 2014 at 12:38

answered Jul 29, 2014 at 12:29

dfsq

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2 Comments

dfsq Over a year ago

@techfoobar Soon there will be another classical way to do it r.foreach(x => console.log(x)) :)

techfoobar Over a year ago

Hmm.. I need to stay upto date on the specs. Thanks. :)

scragar · Accepted Answer · 2014-07-29 12:27:06Z

4

Log cannot be called outside of the console object, you're passing in the function, but not the context.

r.foreach(function(a){
    console.log(a);
});

This works without any issues.

answered Jul 29, 2014 at 12:27

scragar

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2 Comments

DontVoteMeDown Over a year ago

Nice, it works, but why ? In fact it was called with the object console, right ?

scragar Over a year ago

@DontVoteMeDown When you pass a function you pass the function, but not the parent information, calling the function inside your loop then runs into an issue of the this object being the window instead of the console, and the function generates an error.

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