JavaScript parseInt Examples

Convert strings to ints with the parseInt function. Use Number and parseFloat.

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

Conversion syntax. In JavaScript 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 A: We loop over the digits in our string with a for-loop and convert each one to a number.
Part B: We use parseInt, a plus, and the Number constructor. Each string is converted in 3 different ways.
Sum: We sum up all the numbers to show that they are valid digits and get the value 45.
JavaScript program that gets integers from string var codes = "12345"; var sum = 0; // Part A: loop over chars and convert them to ints. for (var i = 0; i < codes.length; i++) { // Part B: 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); // Sum the total. sum += test1 + test2 + test3; } // Write the sum. console.log("SUM: " + sum); Output 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.
JavaScript program that uses parseFloat var test = "100.534"; var resultInt = parseInt(test); console.log("PARSEINT: " + resultInt); var resultFloat = parseFloat(test); console.log("PARSEFLOAT: " + resultFloat) Output 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.
IsNan: We can use the isNaN built-in method to test to see if nothing was parsed by parseInt.
JavaScript program that uses parseInt, isNaN // 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"); } Output PARSEINT: NaN ISNAN

Benchmark, parseInt. Which method should we use to convert strings to ints? Some options, like parseInt and Number, are available. We tested them in a loop with a switch.
Version 1: This version of the code uses the plus operator to convert a digit character into an integer.
Version 2: This version uses the parseInt method, which handles more cases of numeric conversion.
Version 3: Here we use the Number() function to get an integer from a string. We use each integer in a switch.
Result: In a Chromium version from 2019 I found that using Number() is the fastest approach to converting a string to an int.
And: Using parseInt took about twice the time. So avoiding parseInt unless needed is a good approach.
JavaScript program that benchmarks int parsing var c = 0; var codes = "123456"; var x1 = performance.now(); // Version 1: use plus. for (var i = 0; i < 10000000; i++) { for (var z = 0; z < codes.length; z++) { switch (+codes[z]) { case 1: c++; break; case 2: c--; break; case 3: c += 2; break; default: c = 0; break; } } } var x2 = performance.now(); // Version 2: use parseInt. for (var i = 0; i < 10000000; i++) { for (var z = 0; z < codes.length; z++) { switch (parseInt(codes[z])) { case 1: c++; break; case 2: c--; break; case 3: c += 2; break; default: c = 0; break; } } } var x3 = performance.now(); // Version 3: use Number. for (var i = 0; i < 10000000; i++) { for (var z = 0; z < codes.length; z++) { switch (Number(codes[z])) { case 1: c++; break; case 2: c--; break; case 3: c += 2; break; default: c = 0; break; } } } var x4 = performance.now(); // Results. console.log("TIME 1: " + (x2 - x1)); console.log("TIME 2: " + (x3 - x2)); console.log("TIME 3: " + (x4 - x3)); Output TIME 1 (2019): 276.049 Plus TIME 2: 332.764 parseInt TIME 3: 270.944 Number

Notes, parseInt. We have found that using a plus to convert a string to an int is a fast solution. But the fastest approach is to do no conversion at all.
Tip: With some JavaScript changes to how data is stored, you can often have the numbers in their native format already.

A summary. When necessary, a plus sign provides fast string to int conversions in Chromium. Other modern browsers seem to have similar benefits here.


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