`ParseInt`

A Node.js program sometimes has a string but needs an int. For example in numerical operations, ints can be used but strings cannot be.

In Node we can convert a string to an int with parseInt, a built-in method. Another option is with plus (to implicitly convert to an int).

First example

Here we have a string with 5 digits in it. To explore numeric conversions in JavaScript, we surround our parsing statements in a loop.

Part 1 We loop over the digits in our string with a for-loop and convert each one to a number.

Part 2 We use parseInt, a plus, and the Number constructor. Each string is converted in 3 different ways.

Part 3 We sum up all the numbers to show that they are valid digits and get the value 45.

var codes = "12345";
var sum = 0;

// Part 1: loop over chars and convert them to ints.
for (var i = 0; i < codes.length; i++) {
    // Part 2: apply numeric conversion.
    var test1 = parseInt(codes[i]);
    var test2 = +codes[i];
    var test3 = Number(codes[i]);

    // Write our ints.
    console.log("INT: " + test1);
    console.log("INT: " + test2);
    console.log("INT: " + test3);

    // Part 3: sum the total.
    sum += test1 + test2 + test3;
}
// Write the sum.
console.log("SUM: " + sum);INT: 1
INT: 1
INT: 1
INT: 2
INT: 2
INT: 2
INT: 3
INT: 3
INT: 3
INT: 4
INT: 4
INT: 4
INT: 5
INT: 5
INT: 5
SUM: 45

`ParseFloat`

Suppose we have a number with a fractional part like 1.5. This is a floating-point number. We need to use the parseFloat method to retain the fractional part.

Here We use parseInt, which discards everything after the decimal place, and parseFloat.

var test = "100.534";

var resultInt = parseInt(test);
console.log("PARSEINT: " + resultInt);

var resultFloat = parseFloat(test);
console.log("PARSEFLOAT: " + resultFloat)PARSEINT: 100
PARSEFLOAT: 100.534

`IsNaN`

Sometimes a string cannot be parsed—it may contain a word or other text. Here the parseInt method returns NaN, a special value in JavaScript.

Info We can use the isNaN built-in method to test to see if nothing was parsed by parseInt.

// This string cannot be parsed.
var test = "abc";

var result = parseInt(test);
console.log("PARSEINT: " + result);

// We can detect not a number with isNaN.
if (isNaN(result)) {
    console.log("ISNAN");
}PARSEINT: NaN
ISNAN

When necessary, a plus sign provides fast string to int conversions in Node.js. But the parseInt or Number constructs are usually a better choice.