If

最简单的条件判断是if语句，语法是 if(condition){ do this … } 。条件判断为真，才执行分支中的代码。举个字符串的例子：

var country = 'France';
var weather;
var food;
var currency;

if(country === 'England') {
    weather = 'horrible';
    food = 'filling';
    currency = 'pound sterling';
}

if(country === 'France') {
    weather = 'nice';
    food = 'stunning, but hardly ever vegetarian';
    currency = 'funny, small and colourful';
}

if(country === 'Germany') {
    weather = 'average';
    food = 'wurst thing ever';
    currency = 'funny, small and colourful';
}

var message = 'this is ' + country + ', the weather is ' +
            weather + ', the food is ' + food + ' and the ' +
            'currency is ' + currency;

注意: 条件判断可以嵌套。

var name = "John";

if (name === "John") {

}

assert(name === "John");