Written examination notes

Catalog

Preface

Under what circumstances a === a - 1

How to avoid adjacent repetition when randomly selecting array elements ？

How to get integer and decimal parts of a number gracefully ？

Integer

Take decimals

How to correctly judge whether a string is a numeric value ？

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Preface

Unfamiliar language , Continue to learn , Prepare knowledge points first .

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Under what circumstances a === a - 1

1. ±Infinity,Infinity The value of adding and subtracting with any finite number is Infinity

2. A larger number , such as 1e45, To be exact , It's more than Number.MAX_SAFE_INTEGER Number of numbers , It exceeds the integer precision range

3.NaN no way .

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How to avoid adjacent repetition when randomly selecting array elements ？

1. Remember the last selected element , To judge whether or not to repeat .

2. After selection , Temporarily delete the selected element , Select again and then add and delete the next element .

3. Before each choice n-1 term , After selection , Match the selected item with n Item exchange . Select before the first time n Elements .

4. Before each choice n-1 term , After selection , Match the selected item with n Item exchange , Discard the first selected element .

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How to get integer and decimal parts of a number gracefully ？

Integer

1.parseInt(string, radix)`` This method is a method to convert a string to an integer , It has two parameters , The first parameter represents the string to be converted , If the parameter is not a string , Then convert it to a string . The second parameter is the cardinality, i.e. hexadecimal , The default is 10.

2.Math Method ：Math.round It's round ,Math.ceil It's round up ,Math.floor It's rounding down . Positive and negative judgment number , That is floor; Negative namely ceil. Easy way ： Math.trunc, There is no need to judge the positive and negative .

3. utilize Bit or “|” Operation to round （ The limited data length is 32）.

4. Yes 1 Subtract after taking the mold .

Take decimals

1. Get an integer and subtract .

2. Yes 1 modulus .

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How to correctly judge whether a string is a numeric value ？

be used isNumeric To judge the legitimacy of user input .

function isNumeric(obj) {
  return !Number.isNaN(parseFloat(obj))
    && Number.isFinite(Number(obj));
}