贴上jQuery的源码

    isNumeric: function( obj ) {

        // As of jQuery 3.0, isNumeric is limited to
        // strings and numbers (primitives or objects)
        // that can be coerced to finite numbers (gh-2662)
        var type = jQuery.type( obj );
        return ( type === "number" || type === "string" ) &&

            // parseFloat NaNs numeric-cast false positives ("")
            // ...but misinterprets leading-number strings, particularly hex literals ("0x...")
            // subtraction forces infinities to NaN
            !isNaN( obj - parseFloat( obj ) );
    },

为什么需要obj - parseFloat( obj )而不是直接obj - obj
是为了排除什么情况？

在问题补充回答。
空字符串的情况