面試題：實現 (5).add(3).minus(2) 功能

Javascript 浮點數陷阱及解法 github.com/camsong/blo…

Number.MAX_SAFE_DIGITS = Number.MAX_SAFE_INTEGER.toString().length-2
Number.prototype.digits = function(){
	let result = (this.valueOf().toString().split('.')[1] || '').length
	return result > Number.MAX_SAFE_DIGITS ? Number.MAX_SAFE_DIGITS : result
}
Number.prototype.add = function(i=0){
	if (typeof i !== 'number') {
        	throw new Error('請輸入正確的數字');
    	}
	const v = this.valueOf();
	const thisDigits = this.digits();
	const iDigits = i.digits();
	const baseNum = Math.pow(10, Math.max(thisDigits, iDigits));
	const result = (v * baseNum + i * baseNum) / baseNum;
	if(result>0){ return result > Number.MAX_SAFE_INTEGER ? Number.MAX_SAFE_INTEGER : result }
	else{ return result < Number.MIN_SAFE_INTEGER ? Number.MIN_SAFE_INTEGER : result }
}
Number.prototype.minus = function(i=0){
	if (typeof i !== 'number') {
        	throw new Error('請輸入正確的數字');
    	}
	const v = this.valueOf();
	const thisDigits = this.digits();
	const iDigits = i.digits();
	const baseNum = Math.pow(10, Math.max(thisDigits, iDigits));
	const result = (v * baseNum - i * baseNum) / baseNum;
	if(result>0){ return result > Number.MAX_SAFE_INTEGER ? Number.MAX_SAFE_INTEGER : result }
	else{ return result < Number.MIN_SAFE_INTEGER ? Number.MIN_SAFE_INTEGER : result }
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