為什麼 JavaScript 中'1”1 結果為'11”，而'1”- 1 結果為 0？

JavaScript's Unique Handling of String and Number Operators

In JavaScript, the behavior of arithmetic operators like + and - can vary depending on the data types involved. This can be particularly puzzling when working with strings and numbers.

Discrepancy between String Concatenation and Numeric Subtraction

Consider the following code:

console.log("1" + 1);
console.log("1" - 1);

Surprisingly, the first line prints "11" while the second line prints 0. This seemingly contradictory behavior can be explained by understanding JavaScript's handling of these operators.

String Concatenation (with +)

When the + operator is used with strings, it performs string concatenation. In our example, "1" is a string, so JavaScript converts the numeric 1 to a string as well. This results in the concatenation of "1" and "1", producing "11".

Numeric Subtraction (with -)

Unlike string concatenation, numeric subtraction is only applicable to numbers. "1" is a string in our example, so JavaScript attempts to convert it to a number before performing the subtraction. However, strings are not recognized as valid numbers in JavaScript. Consequently, "1" is treated as 0 during the subtraction, resulting in "1" - 1 = 0.

Conclusion

JavaScript's handling of string and number operators can be counterintuitive at times. It is essential to understand the rules governing these operators, especially when working with mixed data types, to avoid unexpected behavior in your code.

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