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The question is a little vague, but right away I tried to break it and was successful with this

function isInteger(num){
 var numCopy = parseFloat(num);
 return !isNaN(numCopy) && numCopy == numCopy.toFixed();
}

isInteger(.9999999999999999999998/.9999999999999999999999);
true

probably because of loss of precision, as @Gallant noted the decimals are getting rounded to zero

Depending on what you are doing this is probably sufficiently accurate.

The question is a little vague, but right away I tried to break it and was successful with this

function isInteger(num){
 var numCopy = parseFloat(num);
 return !isNaN(numCopy) && numCopy == numCopy.toFixed();
}

isInteger(.9999999999999999999998/.9999999999999999999999);
true

probably because of loss of precision, as @Gallant noted the decimals are getting rounded to zero

Depending on what you are doing this is probably sufficiently accurate.

The question is a little vague, but right away I tried to break it and was successful with this

function isInteger(num){
 var numCopy = parseFloat(num);
 return !isNaN(numCopy) && numCopy == numCopy.toFixed();
}

isInteger(.9999999999999999999998/.9999999999999999999999);
true

probably because it overflowed and became ∞/∞ equating to 1.

Depending on what you are doing this is probably sufficiently accurate.

The question is a little vague, but right away I tried to break it and was successful with this

function isInteger(num){
 var numCopy = parseFloat(num);
 return !isNaN(numCopy) && numCopy == numCopy.toFixed();
}

isInteger(.9999999999999999999998/.9999999999999999999999);
true

probably because of loss of precision, as @Gallant noted the decimals are getting rounded to zero

Depending on what you are doing this is probably sufficiently accurate.