How to force JavaScript to do math instead of putting two strings together?

JavaScript often concatenates when you expect addition because the + operator works differently with strings and numbers. Here are several methods to force mathematical addition instead of string concatenation.

The Problem

When JavaScript encounters the + operator with strings, it performs concatenation instead of addition:

console.log("3" + "5");     // "35" (concatenation)
console.log(3 + 5);         // 8 (addition)
console.log("3" + 5);       // "35" (string wins, concatenation)

35
8
35

Using the Unary Plus Operator (+)

The unary + operator converts strings to numbers before addition:

const arr = ['3', '3', '55', '23', '67', '43', '12', '67', '87', '12'];

const sumWithUnaryPlus = arr => {
    return arr.reduce((acc, val) => {
        return acc + (+val || 0);
    }, 0);
};

console.log(sumWithUnaryPlus(arr));

372

Using Number() Constructor

Convert strings explicitly to numbers using Number():

const arr = ['3', '3', '55', '23', '67'];

const sumWithNumber = arr => {
    return arr.reduce((acc, val) => {
        return acc + Number(val);
    }, 0);
};

console.log(sumWithNumber(arr));

151

Using parseInt() or parseFloat()

Use parseInt() for integers or parseFloat() for decimal numbers:

const stringNumbers = ['10', '20.5', '30'];

const sumWithParseInt = stringNumbers.map(str => parseInt(str)).reduce((a, b) => a + b, 0);
const sumWithParseFloat = stringNumbers.map(str => parseFloat(str)).reduce((a, b) => a + b, 0);

console.log("parseInt sum:", sumWithParseInt);     // Truncates decimals
console.log("parseFloat sum:", sumWithParseFloat); // Keeps decimals

parseInt sum: 60
parseFloat sum: 60.5

Simple Addition with Type Coercion

Subtract zero to force numeric conversion:

const a = "15";
const b = "25";

console.log(a + b);           // "1525" (concatenation)
console.log((a - 0) + (b - 0)); // 40 (addition)
console.log(a * 1 + b * 1);   // 40 (multiplication by 1 converts to number)

1525
40
40

Comparison of Methods

Conclusion

Use the unary + operator or Number() constructor for reliable string-to-number conversion. Both handle decimals and provide consistent results when forcing mathematical operations in JavaScript.