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Best practice to use the strict equal sign and the loose equal sign?

medini_1064887566

Hi scripting experts,

I want to know the best practice of using strict equality and loose equality.

When should I use the strict equal signs (`===` and `!==`) and the loose equal signs (`==` and `!=`)?

For example,

var a = prompt(‘a’,’a’);
console.log(a);
console.log(a === 'a'); //false
console.log(a == 'a'); //true

How can I use `===` in the above example? I thought using strict equality can avoid bugs so I want to use strict equal sign if possible.

Thank you very much.

Ka-lip

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Accepted answers

medini_1023909254

see the discussion here: https://stackoverflow.com/questions/359494/which-equals-operator-vs-should-be-used-in-javascript-comparisons

Eckhardt

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medini_1023909254

see the discussion here: https://stackoverflow.com/questions/359494/which-equals-operator-vs-should-be-used-in-javascript-comparisons

Eckhardt

medini_1064887566

Thank you Eckhardt.

In short, the `===` compares whether they are the same object (like `a is b` in Python), and `==` compares the content (like `a == b` in Python).

So in my case, there is no possibility to use `===` to compare different objects and get true treturn.

medini_1024169437

I think that conclusion is not completely right Ka-lip. === is still a comparison operator, the two operants dont have to be exactly the same objects but they need to be of the same type. To illustrate you may add one more line:

var a = prompt('a','a');
console.log("{0} {1}", a, typeof a);
console.log(a === 'a'); //false
console.log(a == 'a'); //true
console.log('' + a === 'a'); // true !!

The point is that in our world we constantly mix JavaScript and Java objects, especially as in your case the JS strings and Java String objects. The prompt call returns a Java String objects which you cant simply compare with === with a JS string, but with == it works as a Java string can be easily converted into a JS one.

So my takeaway is: you can try to use === as oftne as posisble (to avoid the evil twin as described in the article) but that implies that you have to deal with type conversion all the time yourself. And frankly speaking, the type conversion has its advantages :-)

Jan