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parseInt() is not a method of the String prototype but rather a built-in function.
Update this line:
this.currentNum = this.output.parseInt();
to this:
this.currentNum = parseInt(this.output);
And it is a good idea to specify the radix in the second parameter – perhaps 10 in most all cases, for decimal numbers:
this.currentNum = parseInt(this.output, 10);
That way, if a value like 022
is entered, it isn’t interpreted as the octal value (I.e. Decimal number 18
).
Source:stackexchange.com