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Interview questions for frontend developer
https://github.com/artemdemo/frontend-interview-questions/blob/master/js-questions.md
Q. You have variable that may or may not be a number. Write function isNumber() that will return true if it is in fact number or false otherwise.
A.
function isNumber(str) {
return Number(str) == Number(str);
}
see the 20.1.1.1, step 2 of ECMA Specification
When Number is called with argument value, the following steps are taken:
- If no arguments were passed to this function invocation, let n be +0.
2. Else, let n be ? ToNumber(value).
Number constructor try value (argument) type conversion to number type, if it be possible
so realy Boolean, empty Array, empty string and other cases by weak comparison rules - this function take positive result, but it is wrong.
from the condition of question
hat will return true if it is in fact number or false otherwise.
realy rigth answer is
function isNumber(arg) { return isNaN(arg) ? false : typeof arg === 'number' ? true : false }
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