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January 10, 2017 09:52
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1.10 Pure functions
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| //Definition of pure fuctions | |
| // 1. Given the same input, will always return the same output. | |
| // 2. Produces no side effects. | |
| // 3. Relies on no external state. | |
| //pure functions | |
| let array1 = [1,2,3,4,5]; | |
| // pure: not mess up with original data | |
| array1.slice(0,3); | |
| //=> [1,2,3] | |
| array1.slice(0,3); | |
| //=> [1,2,3] | |
| array1.slice(0,3); | |
| //=> [1,2,3] | |
| // not pure: change the value of original data | |
| array1.splice(0,3); | |
| //=> [1,2,3] | |
| array1.splice(0,3); | |
| //=> [4,5] | |
| array1.splice(0,3); | |
| //=> [] | |
| // Not pure | |
| let minimum = 21; | |
| let checkAge = (age)=> { | |
| return age >= minimum; | |
| }; | |
| // The result depends on system state, it adds in external variable and increases cognitive load | |
| // pure | |
| let checkAge2 = (age)=> { | |
| let minimum = 21; //minimum is not an immutable variable | |
| return age >= minimum; | |
| }; | |
| //reason: | |
| //1. Cacheable纯函数总能够根据输入来做缓存 | |
| //2. 可移植性／自文档化（Portable / Self-Documenting） | |
| //纯函数是完全自给自足的，它需要的所有东西都能轻易获得 | |
| //3. 可测试性（Testable） | |
| //4. 合理性（Reasonable）引用透明性（referential transparency） | |
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