javascript - One line of code writing for underscore.js source code

Question

  _.isNaN = function(obj) {
        return _.isNumber(obj) && isNaN(obj); //_.isNumber ？
    }

• _.isNumber is not defined in other parts. According to the && operator, the _.isNumber method is not defined, so _.isNumber(obj) is false, so what is returned is isNaN( obj), why not just return isNaN(obj)?

Answer 1

I found the point. . . I saw it wrong, it turned out to be here

 // Add some isType methods: isArguments, isFunction, isString, isNumber, isDate, isRegExp, isError, isMap, isWeakMap, isSet, isWeakSet.
    _.each(['Arguments', 'Function', 'String', 'Number', 'Date', 'RegExp', 'Error', 'Symbol', 'Map', 'WeakMap', 'Set', 'WeakSet'], function(name) {
        _['is' + name] = function(obj) {
            return toString.call(obj) === '[object ' + name + ']';
        };
    });