var s = null;
var len = s.length; // TypeError：null变量没有length属性

int fd = open("/path/to/file", O_RDONLY);
if (fd == -1) {
    printf("Error when open file!");
} else {
    // TODO
}

try ... catch ... finally

'use strict';
var r1, r2, s = null;
try {
    r1 = s.length; // 此处应产生错误
    r2 = 100; // 该语句不会执行
} catch (e) {
    alert('出错了：' + e);
} finally {
    console.log('finally');
}
console.log('r1 = ' + r1); // r1应为undefined
console.log('r2 = ' + r2); // r2应为undefined

try {
    ...
} catch (e) {
    ...
} finally {
    ...
}

try {
    ...
} catch (e) {
    ...
}

try {
    ...
} finally {
    ...
}

错误类型

Error派生的TypeError、ReferenceError等错误对象

try {
    ...
} catch (e) {
    if (e instanceof TypeError) {
        alert('Type error!');
    } else if (e instanceof Error) {
        alert(e.message);
    } else {
        alert('Error: ' + e);
    }
}

抛出错误

'use strict';
var r, n, s;
try {
    s = prompt('请输入一个数字');
    n = parseInt(s);
    if (isNaN(n)) {
        throw new Error('输入错误');
    }
    // 计算平方:
    r = n * n;
    alert(n + ' * ' + n + ' = ' + r);
} catch (e) {
    alert('出错了：' + e);
}

var n = 0, s;
try {
    n = s.length;
} catch (e) {
    console.log(e);
}
console.log(n);