Require or disallow Yoda Conditions (yoda)

要求或者禁止Yoda条件 (yoda)

The --fix option on the command line can automatically fix some of the problems reported by this rule.

命令行中的 --fix 选项可以自动修复一些该规则报告的问题。

Yoda conditions are so named because the literal value of the condition comes first while the variable comes second. For example, the following is a Yoda condition:

Yoda 条件被如此命名，是因为在条件判断中字面量在先而变量在第二的位置。例如，以下是 Yoda 条件：

if ("red" === color) {
    // ...
}

This is called a Yoda condition because it reads as, “if red equals the color”, similar to the way the Star Wars character Yoda speaks. Compare to the other way of arranging the operands:

它被叫做 Yoda 条件是因为它这样读：”红色是颜色”，类似于星球大战中 Yoda 的讲话方式。对比另一种操作数的排序方式：

if (color === "red") {
    // ...
}

This typically reads, “if the color equals red”, which is arguably a more natural way to describe the comparison.

这通常读作，”颜色是红的”，这是一种更自然的方式去描述对比。

Proponents of Yoda conditions highlight that it is impossible to mistakenly use = instead of == because you cannot assign to a literal value. Doing so will cause a syntax error and you will be informed of the mistake early on. This practice was therefore very common in early programming where tools were not yet available.

Yoda 条件的支持者强调，错误地使用 = 代替 == 是不可能的，因为你不能分配给一个文本值。这样做将导致一个语法错误，并且你会被提早告知。因此在工具尚不可用的早期编程中，这种做法是非常常见的。

Opponents of Yoda conditions point out that tooling has made us better programmers because tools will catch the mistaken use of = instead of == (ESLint will catch this for you). Therefore, they argue, the utility of the pattern doesn’t outweigh the readability hit the code takes while using Yoda conditions.

Yoda 条件的反对者指出工具使我们成为更好的程序员，因为工具将捕获使用 = 代替 == 的错误（ESLint 将为你捕获这个错误）。因此,他们认为当使用 Yoda 条件时，该模式的实用性并不高于代码的可读性。

Rule Details

This rule aims to enforce consistent style of conditions which compare a variable to a literal value.

这条规则需要一个参数。如果是 "never" 那么比较绝不能是一个 Yoda 条件。

Options

This rule can take a string option:

该规则只有一个字符串选项：

If it is the default "never", then comparisons must never be Yoda conditions.
如果是默认的 "never"，则比较绝不能是 Yoda 条件。
If it is "always", then the literal value must always come first.
如果是 "always"，那么字面量值必须放在首位。

The default "never" option can have exception options in an object literal:

默认选项 "never" 可以有例外情况，是个对象：

If the "exceptRange" property is true, the rule allows yoda conditions in range comparisons which are wrapped directly in parentheses, including the parentheses of an if or while condition. The default value is false. A range comparison tests whether a variable is inside or outside the range between two literal values.
如果 "exceptRange" 为 true，该规则允许 yoda 条件出现在被括号包裹的范围比较中，包括 if 或 while 条件的括号。默认为 false。一个范围比较测试一个变量是在两个字面量内部还是外部。
If the "onlyEquality" property is true, the rule reports yoda conditions only for the equality operators == and ===. The default value is false.
如果 "onlyEquality" 为 true，该规则只对等号操作符 == 和 === 报告 yoda 条件。默认为 false。

The onlyEquality option allows a superset of the exceptions which exceptRange allows, thus both options are not useful together.

onlyEquality 选项所允许的是 exceptRange 所允许的例外的超集，因此同时使用它们什么用。

never

Examples of incorrect code for the default "never" option:

默认选项"never"的错误代码示例：

/*eslint yoda: "error"*/

if ("red" === color) {
    // ...
}

if (true == flag) {
    // ...
}

if (5 > count) {
    // ...
}

if (-1 < str.indexOf(substr)) {
    // ...
}

if (0 <= x && x < 1) {
    // ...
}

Examples of correct code for the default "never" option:

默认选项 "never" 的正确代码示例：

/*eslint yoda: "error"*/

if (5 & value) {
    // ...
}

if (value === "red") {
    // ...
}

exceptRange

Examples of correct code for the "never", { "exceptRange": true } options:

选项 "never", { "exceptRange": true } 的正确代码示例：

/*eslint yoda: ["error", "never", { "exceptRange": true }]*/

function isReddish(color) {
    return (color.hue < 60 || 300 < color.hue);
}

if (x < -1 || 1 < x) {
    // ...
}

if (count < 10 && (0 <= rand && rand < 1)) {
    // ...
}

function howLong(arr) {
    return (0 <= arr.length && arr.length < 10) ? "short" : "long";
}

onlyEquality

Examples of correct code for the "never", { "onlyEquality": true } options:

选项 "never", { "onlyEquality": true } 的正确代码示例：

/*eslint yoda: ["error", "never", { "onlyEquality": true }]*/

if (x < -1 || 9 < x) {
}

if (x !== 'foo' && 'bar' != x) {
}

always

Examples of incorrect code for the "always" option:

选项 "always" 的错误代码示例：

/*eslint yoda: ["error", "always"]*/

if (color == "blue") {
    // ...
}

Examples of correct code for the "always" option:

选项 "always" 的正确代码示例：

/*eslint yoda: ["error", "always"]*/

if ("blue" == value) {
    // ...
}

if (-1 < str.indexOf(substr)) {
    // ...
}

Version

This rule was introduced in ESLint 0.7.1.

该规则在 ESLint 0.7.1 中被引入。