JavaScript arithmetic operations between strings of numbers

Question

I'm wondering what happens behind the scenes when we either add, subtract or multiply two strings of numbers. Here is an example:

let strNum1 = "300";
let strNum2 = "22";

let multiply = function(num1, num2) {
    let product = num1 * num2;
    
    return `${product}`
      
};

multiply(strNum1, strNum2); //this will return ==> "6600"

Does the JS engine turns these into integers first and then performs the operations or does it know "magically" that they are numbers even though it's in a string form? The reason I'm asking is because of the long multiplication algorithm. For numbers bigger than 8 chars it becomes funky when multiplying with the operator vs using the algorithm.

This is a leetcode question btw.

The * operator coerces the strings into numbers, that'll happen with -, / ,%, etc. + is the exception it'll be interpreted as concatenating of two strings. You can still use + to coerce string into number by enclosing the right side string in parenthesis: "1" +(+"2") — zer00ne
– zer00ne, Commented Jun 11, 2022 at 3:54
@habibg I added an answer. Did you get a chance to look into that ? I hope that will help you in understanding the use case and will work as per your requirement. — Rohìt Jíndal
– Rohìt Jíndal, Commented Jun 16, 2022 at 5:56

Rohìt Jíndal · Accepted Answer · 2022-06-13 05:53:24Z

1

In case of addition (+), When a number is added to a string, JavaScript converts the number to a string before concatenation but in case of other arithmetic operations like *, -, / JS engine implicitly convert the string into integer.

Demo :

let result;

// numeric string used with + gives string type

result = '3' + '2'; 
console.log(result, typeof result) // "32", "string"

// numeric string used with - , / , * results number type

result = '3' * '2'; 
console.log(result, typeof result) // 6, "number"

result = '3' - '2'; 
console.log(result, typeof result) // 1, "number"

result = '3' / '2'; 
console.log(result, typeof result) // 1.5, "number"

answered Jun 13, 2022 at 5:53

Rohìt Jíndal

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Comments

thebyteland · Accepted Answer · 2022-06-11 03:31:04Z

0

You can parse your values before and after operation:

let multiply = function(num1 = '', num2 = '') {
    const product = Number(num1) * Number(num2);
    
    return `${product}` // Or String(product)
};

answered Jun 11, 2022 at 3:31

thebyteland

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Son Nguyen · Accepted Answer · 2022-06-11 03:45:58Z

0

To answer your question: The JS Engine turns those strings into integers before doing the arithmetic operation. It is called Implicit coercion.

You can read more about that in this You Don't Know JS Chapter.

answered Jun 11, 2022 at 3:45

Son Nguyen

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