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Where Javascript Coercion went wrong?

aloksdiptim profile image alokz diptim! ・2 min read

Coercion is unknowingly the conversion of a datatype to another with the use of the Javascript engine. One of the few surprises in Javascript. Now, this behaves differently for various operators and data types such as:

  • + Operator
  • - Operator
  • Boolean
  • Equality signs
  • Arrays

It happens mostly because of the dynamic type declaration of javascript variable. The use of var.

With the use of other programming languages such as c#, If we make use of numbers and strings as data types, then, compile-time ought to show us the error before runtime.

So let’s dive in into each instance of Javascript coercion:

  1. From practice, the + operator returns strings, so the Javascript coerce the operands to strings. Example: 3 + “90” will return 390
  2. Negative sign acts as a number, and it coerces the operand to a number instead Example: 90 - “3” will return 87
  3. Boolean, yes!
    • The or/and logic gates
    • Apple || “orange” returns Apple which is the first value for the or operator and also because it’s true.
    • If it were (undefined| “”| 0| false) || “orange” then orange would be returned.
    • So you can write a function such as this: Function Whatsup(nickname){ Nickname = nickname || “Stranger.”; console.log(“Hey ” + nickname); } Whatsup(); Since nickname would be undefined and then false, the console will print out “Hey Stranger.”
    ->> and for the “and” operator; It returns the second value because it’s always false;
  4. Equality checks for values and types == checks for values, so coercion still occurs. If (44 == “44”) will return true but if the triple operator equality sign is used instead, it will return false. Ex:If (44 === “44”) returns false;
  5. The Array will return Nan for the - operator Ex; [“Right Now”] - [“Later”] will return Nan(Not a Number) While [“Right Now”] + [“Later”] will concatenate and return RightNowLater as a string

Remember the + operator always return strings as their value.

So in an instance where we have:

console.log(1 < 4 < 3); it will return true instead of false because the javascript sees it as:
console.log(true < 3);
“true” would be coerced to 1 and when compared will be 1 < 3 which will be true.

Here are some of the weird behaviours of coercion.

They are best tackled with strict equality operator sign, ===

But then again, coercion may not only be with the comparison but also how javascript checks for a boolean value.
To check how Javascript coerce output, check out Number(“”), Number(), or Boolean(undefined) and see what it coerce to with the given results.
Ex: I can check,

boolean("0"); that will be coerce to false
Boolean(undefined); will be coerce to false
Number("3") will be coerce to 3;
Number("Hello") will be coerce to NaN

Also checking the precedence matters as well.

Thanks and that’s all about Coercion.

If you love/like, please share/comment. Thanks!

Posted on Jan 2 '19 by:

aloksdiptim profile

alokz diptim!

@aloksdiptim

Software Developer, Scientist and Graphical artist

Discussion

markdown guide
 

console.log(1 < 4 < 3); it will return true instead of false because the javascript sees it as:
console.log(true < 3);
“true” would be coerced to 1 and when compared will be 1 < 3 which will be true.

I never knew that. I had always thought it was just a quirk of JavaScript. Thanks for clearing things up!

 

Yes, it is because of coercion.

Thanks for reading the post.