You need to pass the options as the last parameter (for .diff there are 3 parameters)

Copyvar jsonDiff = require('json-diff')
console.log(jsonDiff.diff({ foo: 'bar' }, { foo: 'baz' }, {keysOnly: true}));

For diffString there are four params (the 3rd being the colorize options, and the 4th the options)

Copyvar jsonDiff = require('json-diff')
console.log(jsonDiff.diffString({ foo: 'bar' }, { foo: 'baz' }, undefined, {keysOnly: true}));

Friends! I'm really sorry if I don't understand something, or explain question in wrong terms. All I wanna do is to compare the equality of two pieces of JSON-standardized data like this:

{"skip":0, "limit":7, "arr": [1682, 439, {"x":2, arr:[]}] }

{"skip":0, "limit":7, "arr": [1682, 450, "a", ["something"] }

I'm 100% sure there will be no functions, Date, null or undefined, etc. in these data. I want to say I don't want to compare JavaScript objects in the most general case (with complex prototypes, circular links and all this stuff). The prototype of these data will be Object. I'm also sure lots of skillful programmers have answered this question before me.

The main thing I'm missing is that I don't know how to explain my question correctly. Please feel free to edit my post.

My answer:

First way: Unefficient but reliable. You can modify a generic method like this so it does not waste time looking for functions and undefined. Please note that generic method iterates the objects three times (there are three for .. in loops inside)

Second way: Efficient but has one restriction. I've found JSON.stringify is extremely fast in node.js. The fastest solution that works is:

JSON.stringify(data1) == JSON.stringify(data2)

Very important note! As far as I know, neither JavaScript nor JSON don't matter the order of the object fields. However, if you compare strings made of objects, this will matter much. In my program the object fields are always created in the same order, so the code above works. I didn't search in the V8 documentation, but I think we can rely on the fields creation order. In other case, be aware of using this method.

In my concrete case the second way is 10 times more efficient then the first way.