Category: JavaScript Gotchas

Declaring a variable as a number before adding to it with +=

Today I learned… that you have to explicitly declare a number variable as a number before you can increment it with += in JavaScript.

This is probably a boneheaded newbie mistake, but it had me stumped for several minutes. I declared a variable, var total; and then tried to add to it by writing total +=5.  And then I scratched my head as my code returned NaN.

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var total;
 
total += 5;
console.log(total); //prints "NaN"

This happens because “undefined” and zero are not equivalent in JavaScript! 

Even though JavaScript isn’t “strongly typed”, you can’t add numbers to something that isn’t a number yet.

Declaring total by setting it to an actual number, ie: var total = 0, fixed my “NaN” problem.

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var total = 0;
 
total += 5;
console.log(total); //prints a 5