Check if variable is supported by browser using JavaScript

Question

There are some functions that I would like to use in my JavaScript code, but a few of them aren't supported on all browsers. Is there a way to check if the function is available in JavaScript?

For example:

/* context.measureText().actualBoundingBoxAscent is
not supported in Internet Explorer but is on almost
every other browser. */

if (variableExists(ctx.measureText('').actualBoundingBoxAscent)) == true) {
    // do code that uses actualBoundingBoxAscent
}
else {
    // do back up code if actualBoundingBoxAscent doesn't exist
}