与ES6反应的类变量 [英] Class variables in React with ES6
问题描述
这个问题可能已在其他地方得到解答,但在标记为重复之前,请帮助我。我指的是使用react和d3的以下codepen:
...如果你点击旁边的齿轮图标,你会看到Babel被选为预处理器。
在这种特殊情况下,Babel采用了这个:
class颜色扩展组件{
colors = d3.schemeCategory20;
width = d3.scaleBand()
.domain(d3.range(20));
// ....
}
.. 。并且好像它是这样编写的:
class颜色扩展组件{
constructor(){
this.colors = d3.schemeCategory20;
this.width = d3.scaleBand()
.domain(d3.range(20));
}
// ....
}
。 ..(然后可能会将其转换为符合ES5的代码,具体取决于转换设置)。
This question might have been answered somewhere else, but before marking as duplicate, please help me with it. I am referring to the following codepen using react and d3: https://codepen.io/swizec/pen/oYNvpQ
However, I am not able to figure out how can this codepen work with variables declared inside the class without any keywords:
class Colors extends Component {
colors = d3.schemeCategory20;
width = d3.scaleBand()
.domain(d3.range(20));
....
}
When I try to execute this code, I get the following error:
./app/components/D3IndentedTree/Chloreophath.js
Module build failed: SyntaxError: Unexpected token (13:11)
11 | // Draws an entire color scale
12 | class Colors extends Component {
> 13 | colors = d3.schemeCategory20;
| ^
14 | width = d3.scaleBand()
15 | .domain(d3.range(20));
16 |
I am not able to figure out why am I getting this error. I understand that you cant declare variables/properties of class directly inside the class. But how come then the code pen is working?
Thanks in advance!
Update: Adding the webpack.config.js file:
var path = require('path');
var webpack = require('webpack');
var HtmlWebpackPlugin = require('html-webpack-plugin');
module.exports = {
entry: './app/index.js',
output: {
path: path.resolve(__dirname, 'dist'),
filename: 'index_bundle.js',
publicPath: '/'
},
module: {
rules: [
{ test: /\.(js)$/, use: 'babel-loader' },
{ test: /\.css$/, use: [ 'style-loader', 'css-loader'] },
{
test: /\.png$/,
loader: "url-loader?limit=100000"
},
{
test: /\.jpg$/,
loader: "file-loader"
},
{
test: /\.(woff|woff2)(\?v=\d+\.\d+\.\d+)?$/,
loader: 'url-loader? limit=10000&mimetype=application/font-woff'
},
{
test: /\.ttf(\?v=\d+\.\d+\.\d+)?$/,
loader: 'url-loader?limit=10000&mimetype=application/octet-stream'
},
{
test: /\.eot(\?v=\d+\.\d+\.\d+)?$/,
loader: 'file-loader'
},
{
test: /\.svg(\?v=\d+\.\d+\.\d+)?$/,
loader: 'url-loader?limit=10000&mimetype=image/svg+xml'
}
]
},
plugins: [new HtmlWebpackPlugin({
template: 'app/index.html'
}),
new webpack.ProvidePlugin({
"React": "react",
}),],
devServer: {
historyApiFallback: true
}
};
But how come then the code pen is working?
Because it's using a transpiler (in that case, Babel) that supports that syntax (which is currently a Stage 3 proposal, not a specified feature [yet], but is commonly supported by transpilers used with React code).
You can see that it's transpiling with Babel because it says "(Babel)" next to "JS" in the JS pane's header:
...and if you click the gear icon next to it, you'll see Babel selected as the "Preprocessor".
In this particular case, Babel takes this:
class Colors extends Component {
colors = d3.schemeCategory20;
width = d3.scaleBand()
.domain(d3.range(20));
// ....
}
...and makes it as though it had been written like this:
class Colors extends Component {
constructor() {
this.colors = d3.schemeCategory20;
this.width = d3.scaleBand()
.domain(d3.range(20));
}
// ....
}
...(and then might well turn that into ES5-compliant code, depending on the transpiling settings).
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