最好的方法是:

function isNumber(num) {
  return (typeof num == 'string' || typeof num == 'number') && !isNaN(num - 0) && num !== '';
};

这满足以下测试用例:

assertEquals("ISNUMBER-True: 0", true, isNumber(0));
assertEquals("ISNUMBER-True: 1", true, isNumber(-1));
assertEquals("ISNUMBER-True: 2", true, isNumber(-500));
assertEquals("ISNUMBER-True: 3", true, isNumber(15000));
assertEquals("ISNUMBER-True: 4", true, isNumber(0.35));
assertEquals("ISNUMBER-True: 5", true, isNumber(-10.35));
assertEquals("ISNUMBER-True: 6", true, isNumber(2.534e25));
assertEquals("ISNUMBER-True: 7", true, isNumber('2.534e25'));
assertEquals("ISNUMBER-True: 8", true, isNumber('52334'));
assertEquals("ISNUMBER-True: 9", true, isNumber('-234'));

assertEquals("ISNUMBER-False: 0", false, isNumber(NaN));
assertEquals("ISNUMBER-False: 1", false, isNumber({}));
assertEquals("ISNUMBER-False: 2", false, isNumber([]));
assertEquals("ISNUMBER-False: 3", false, isNumber(''));
assertEquals("ISNUMBER-False: 4", false, isNumber('one'));
assertEquals("ISNUMBER-False: 5", false, isNumber(true));
assertEquals("ISNUMBER-False: 6", false, isNumber(false));
assertEquals("ISNUMBER-False: 7", false, isNumber());
assertEquals("ISNUMBER-False: 8", false, isNumber(undefined));
assertEquals("ISNUMBER-False: 9", false, isNumber(null));

检查值是否为string字面值或string对象:

function isString(o) {
    return typeof o == "string" || (typeof o == "object" && o.constructor === String);
}

单元测试:

function assertTrue(value, message) {
    if (!value) {
        alert("Assertion error: " + message);
    }
}

function assertFalse(value, message)
{
    assertTrue(!value, message);
}

assertTrue(isString("string literal"), "number literal");
assertTrue(isString(new String("String object")), "String object");
assertFalse(isString(1), "number literal");
assertFalse(isString(true), "boolean literal");
assertFalse(isString({}), "object");

检查一个数字类似:

function isNumber(o) {
    return typeof o == "number" || (typeof o == "object" && o.constructor === Number);
}

对于数字检测，以下文章来自Douglas Crockford的JavaScript: the Good Parts:

isFinite函数是确定一个值是否可以用作数字的最佳方法，因为它拒绝NaN和Infinity。不幸的是，isFinite将尝试将其操作数转换为一个数字，因此如果一个值实际上不是一个数字，那么它不是一个好的测试。你可能想要定义自己的isNumber函数:

var isNumber = function isNumber(value) { return typeof value === 'number' &&
            isFinite(value);
};

我发现的最好的方法是检查字符串上的方法，即:

if (x.substring) {
// do string thing
} else{
// do other thing
}

或者如果你想对number属性做一些检查，

if (x.toFixed) {
// do number thing
} else {
// do other thing
}

这有点像“鸭子打字”，由你自己决定哪种方式最有意义。我没有足够的因果报应来评论，但typeof失败的盒装字符串和数字，即:

alert(typeof new String('Hello World'));
alert(typeof new Number(5));

将提醒“对象”。

呃，不如这样:

function IsString(obj) {
    return obj !== undefined && obj != null && obj.toLowerCase !== undefined;
}

经过几个月后的进一步检查，这只能保证obj是一个定义了方法或属性名称toLowerCase的对象。我为我的回答感到惭愧。请参阅投票最多的类型。

简单而彻底:

function isNumber(x) {
  return parseFloat(x) == x
};

测试用例:

console.log('***TRUE CASES***');
console.log(isNumber(0));
console.log(isNumber(-1));
console.log(isNumber(-500));
console.log(isNumber(15000));
console.log(isNumber(0.35));
console.log(isNumber(-10.35));
console.log(isNumber(2.534e25));
console.log(isNumber('2.534e25'));
console.log(isNumber('52334'));
console.log(isNumber('-234'));
console.log(isNumber(Infinity));
console.log(isNumber(-Infinity));
console.log(isNumber('Infinity'));
console.log(isNumber('-Infinity'));

console.log('***FALSE CASES***');
console.log(isNumber(NaN));
console.log(isNumber({}));
console.log(isNumber([]));
console.log(isNumber(''));
console.log(isNumber('one'));
console.log(isNumber(true));
console.log(isNumber(false));
console.log(isNumber());
console.log(isNumber(undefined));
console.log(isNumber(null));
console.log(isNumber('-234aa'));