One, foreword

Recently, a small partner consulted Hu Ge in the official number of this interview question, I thought it was a more interesting interview question, here to share with you small partners. I gave you the answer first, is it the same as you understand? !

[1, 2, 3].map(parseInt) // [1, NaN, NaN]Copy the code

If you know your answer, please go out and turn left: React source code/principle to learn.

Second, the analysis of

The interview question itself is not complicated. Can not answer the question of small partners, most of the focus on parseInt this function do not understand or understand the comprehensive, the following by Brother Hu for you to tell one by one.

1. The map function

The map() method creates a new array, and the result is the result returned when each element in the array calls one of the provided functions.

grammar

let newArray = arr.map(function calback(currentValue[, index[, array]]) {
    // Return element for newArray
}[, thisArg])Copy the code

Callback callback function

Parameter names meaning optional
currentValue The current element being processed in the callback array Will choose
index The index of the current element being processed in the calback array optional
array The array called by the map method optional

thisArg

ThisArg, optional, supports callback when the value is used as this

[1, 2, 3].map((v) => {
    return v * v
})
// [1, 4, 9]Copy the code

2. The parseInt function

ParseInt (string, radix) converts a string string to an integer for radix radix, which is between 2 and 36. Return value: Returns the parsed integer value, or NaN if it cannot be converted to a value

ParseInt (10) // 10 ---- The base 10 string '10' is converted to 10 parseInt(10, 2) // 2 ---- the base 2 string '10' is converted to 2 parseInt(3, 2) // NaN -- there is no 3 in base 2Copy the code

Additional information:

If the cardinality is undefined, or the cardinality is 0, or no cardinality is specified, JavaScript does the following:

If the string string begins with “0x” or “0x”, the cardinality is 16 (hexadecimal).

If the string starts with “0” and the base is either 8 (octal) or 10 (decimal), the implementation environment determines which base. ECMAScript 5 specifies the use of 10, but not all browsers follow this rule. Therefore, the radix parameter values should always be specified.

If the string string begins with any other value, the base is 10 (decimal).

Three, witness the miracle moment

ParseInt (1, 2, 3); parseInt (1, 2, 3); map(parseInt);

[1, 2, 3]. Map ((v, index) => {return parseInt(v, index)}) [parseInt(1, 0), parseInt(2, 1), parseInt(3, 2)] ===> [1, NaN, NaN]Copy the code

Note that parseInt is written as a callback in a way that is easy to understand.

Four, conclusion

[1, 2, 3]. Map (parseInt)

Afterword.

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