A Look at the Difference Between Number and parseInt

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When converting a numeric string into a number,

I had been picking one of the two at random, so it felt like time to sort this out.

#What they share

Number and parseInt are both functions that convert to a number

#How they behave

Number : evaluates the whole value of the argument it receives and converts it to a number

                 `parseInt` : **checks the argument one character at a time, converts only up to the last valid integer**, ignores the rest, and returns a number

( forgiving about whitespace, though not about letters )

#Examples

console.log(Number("123"));        // 123
console.log(Number("12.34"));      // 12.34
console.log(Number("123abc"));     // NaN
console.log(Number("   0012 300  "));   // NaN
console.log(Number(true));         // 1
console.log(Number(false));        // 0
console.log(Number(null));         // 0
console.log(Number(undefined));    // NaN
console.log(Number("Infinity"));   // Infinity

console.log(parseInt("123"));      // 123
console.log(parseInt("12.34"));    // 12
console.log(parseInt("123abc"));   // 123
console.log(parseInt("   0012 300  "));   // 12
console.log(parseInt("abc123"));   // NaN
console.log(parseInt("  42"));     // 42 (whitespace allowed)
console.log(parseInt("101", 2));   // 5 (read as binary)

🤔 Something I wondered about

In an earlier post I wrote that JavaScript treats a string containing e as exponential notation.

Debugging it gives

Number('1e5') // 100,000
parseInt('1e5') // 1

Number checks the entire argument, so the e in the string makes it exponential notation.

parseInt checks one character at a time, so it treats e as a letter and you can see it stops at 1.

So then,

which one should you use to turn a string type like '12345' into a number type?

#Conclusion

When converting a string that contains nothing but digits, Number is the better choice.

parseInt is better when you need a number even though the string has letters in it, and when you want to drop everything after the decimal point.