How Lodash _.multiply Handles Numeric Strings

The Lodash _.multiply method implicitly coerces string inputs containing valid numeric characters into standard JavaScript numbers before performing the calculation. Because Lodash relies on JavaScript’s native arithmetic coercion under the hood, passing strings containing numbers to _.multiply results in a valid numeric product rather than string concatenation or a type error. This article explains how this conversion functions under the hood, how common string scenarios are processed, and how edge cases like whitespace or invalid characters are resolved.

Under the Hood: JavaScript Type Coercion

In Lodash, _.multiply is implemented using an internal helper function called createMathOperation. This helper delegates the multiplication to JavaScript's standard multiplication operator (*).

In JavaScript, the * operator triggers implicit type coercion on operands that are not already numbers. When an operand is a string, JavaScript invokes the abstract operation ToNumber(string). Consequently, _.multiply converts the inputs to native numbers before multiplying them and returns a primitive number as the final output.

Behavior with Valid Numeric Strings

When passing strings that strictly represent valid integers or floating-point numbers, _.multiply converts them without issues:

• Mixed string and number: _.multiply('5', 2) returns 10.
• Both arguments as strings: _.multiply('4', '2.5') returns 10.
• Negative numeric strings: _.multiply('-3', '4') returns -12.
• Scientific notation strings: _.multiply('1e2', '2') returns 200.

In every case, the return value is of the type number, not string.

Edge Cases and Non-Standard Strings

Because _.multiply relies on native ToNumber coercion rules, different types of string inputs yield specific results:

• Strings with Whitespace: Leading and trailing whitespace is automatically trimmed during conversion. For example, _.multiply(' 7 ', ' 3 ') successfully evaluates to 21.
• Empty Strings: An empty string ("") or a string containing only spaces evaluates to 0. Consequently, _.multiply('', 10) returns 0.
• Non-Numeric Strings: If a string contains non-numeric characters that cannot be parsed as a number (such as "5px" or "hello"), JavaScript evaluates the value to NaN (Not-a-Number). As a result, _.multiply('10px', 2) returns NaN. Unlike functions like parseInt(), _.multiply does not extract numbers from mixed alphanumeric strings.