parseInt("123qwe")

returns 123

Number("123qwe")

returns NaN

In other words parseInt() parses up to the first non-digit and returns whatever it had parsed. Number() wants to convert the entire string into a number, which can also be a float BTW.

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EDIT #1: Lucero commented about the radix that can be used along with parseInt(). As far as that is concerned, please see THE DOCTOR's answer below (I'm not going to copy that here, the doc shall have a fair share of the fame...).

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EDIT #2: Regarding use cases: That's somewhat written between the lines already. Use Number() in cases where you indirectly want to check if the given string completely represents a numeric value, float or integer. parseInt()/parseFloat() aren't that strict as they just parse along and stop when the numeric value stops (radix!), which makes it useful when you need a numeric value at the front "in case there is one" (note that parseInt("hui") also returns NaN). And the biggest difference is the use of radix that Number() doesn't know of and parseInt() may indirectly guess from the given string (that can cause weird results sometimes).

Worth noting is that you shouldn't use parseInt to convert a String into a Number. Use var n = Number('123'); instead, or Math.round()/Math.trunc() if you want to convert a floating point number into an integer. Only use parseInt when converting between numbers of different base. semantically, you mostly do not want to parse a string for a number, you want to do a type conversion, therefore Number() makes more sense. parseInt() is much slower to execute (not that it's noticeable). parseInt() doesn't give an error or any warning as long as the first character is a number. parseInt('3asdfghhj') will simply return 3. parseInt() coerces the input to a string without error or warning. parseInt([321,'asdf',window]) returns 321. parseInt() doesn't treat the number Infinity correctly. parseInt(Number.MAX_VALUE * 2) returns NaN, but Number(Number.MAX_VALUE * 2) correctly returns Infinity parseInt() doesn't threat negative exponents correctly. parseInt(2e-6) returns 0 and parseInt(2e-7) returns 2. /rant.

Well, they are semantically different, the Number constructor called as a function performs type conversion and parseInt performs parsing, e.g.:

// parsing:
parseInt("20px");       // 20
parseInt("10100", 2);   // 20
parseInt("2e1");        // 2

// type conversion
Number("20px");       // NaN
Number("2e1");        // 20, exponential notation

Also parseInt will ignore trailing characters that don't correspond with any digit of the currently used base.

The Number constructor doesn't detect implicit octals, but can detect the explicit octal notation:

Number("010");         // 10
Number("0o10")         // 8, explicit octal

parseInt("010");       // 8, implicit octal
parseInt("010", 10);   // 10, decimal radix used

And it can handle numbers in hexadecimal notation, just like parseInt:

Number("0xF");   // 15
parseInt("0xF"); //15

In addition, a widely used construct to perform Numeric type conversion, is the Unary + Operator (p. 72), it is equivalent to using the Number constructor as a function:

+"2e1";   // 20
+"0xF";   // 15
+"010";   // 10

None of them will work in those cases because you are passing the element reference (document.getElementById(‘number’)) to the functions, but a string is expected. For them to work you should access the element content (for example with .innerHTML): let stringNumber = document.getElementById('numb…

ParseInt is meant to change a string value that is a number into an actual number value. JavaScript console.log(parseInt("3")); // output will be the number 3 not a string So this would come in handy when you are looking to use numbers gathered from an input value from the user in a form to do math, like in a calculator. So the number coming in a a string "37820" and to do math on it you need it to be a number 37820 and parseInt() will do that.

Number can also return floats. parseInt only returns integers and lets you define a radix.