Rendall's Blog

  1. Problem: Typescript adds Object.defineProperty(exports, "__esModule", { value: true });

    You're using Typescript in a frontend project, and import a module into your code. Something like:
    import { Observable } from 'rxjs';

    You're using a bundler like Webpack that squeezes those modules into a single minimized javascript file, so you want to keep that line as-is.

    However, when Typescript compiles into javascript, it adds the line Object.defineProperty(exports, "__esModule", { value: true }); which causes an error Uncaught ReferenceError: exports is not defined or something similar in your browser's console.

    You search for solutions, but everyone is talking about using RequireJS like it's 2014!

    You want to get rid of this unwanted Object.defineProperty line but leave the rest of your code untouched.

    Solution: Modify these two lines in your tsconfig.json to the following:
    "module": "es6",
    "lib": ["dom","es6"]

    Explanation:

    • commonjs (the current Typescript default) or one of the other suggested options assumes you are writing a Node.js application, or
    • that you will use a module loader (like RequireJS) which loads files into your web application at runtime.
    • es6 is a working standard that will keep your import line intact.
    • But es6 assumes you are importing from a javascript file (e.g. import { this } from './that.js').
    • However, you are using a package manager, you hipster, and that module is not in an external javascript file, but is sitting in a node_modules directory somewhere waiting for your package bundler to roll it on up.
    • Typescript sees the es6 module style, but not the es6 lib, and so believes you are loading an external javascript file. This is why you need to add the es6 to lib:
    • But once you enable lib:, the browser-specific code in your typescript will break without the dom library, so you need to add that, too.

    Step-by-step: Still need help? Follow these instructions:

    1. If you do not have a tsconfig.json: In your project directory, type tsc --init
    2. In the tsconfig.json file, find the line starting module: and change its value to es6 like so: "module": "es6"
    3. Further down, find the line starting lib: and add dom and es6 to the array, like so: "lib": ["dom","es6"]. Make sure to uncomment it if necessary.

    More info:

    • The tsconfig.json schema contains all of the options for every key-value pair, including:
    • Current module options (one of): "commonjs", "amd", "umd", "system", "es6", "es2015", "esnext", or "none"
    • Current lib options (any combination of): "es5", "es6", "es2015", "es7", "es2016", "es2017", "es2018", "esnext", "dom", "dom.iterable", "webworker", "scripthost", "es2015.core", "es2015.collection", "es2015.generator", "es2015.iterable", "es2015.promise", "es2015.proxy", "es2015.reflect", "es2015.symbol", "es2015.symbol.wellknown", "es2016.array.include", "es2017.object", "es2017.intl", "es2017.sharedmemory", "es2017.string", "es2017.typedarrays", "es2018.intl", "es2018.promise", "es2018.regexp", "esnext.asynciterable", "esnext.array", "esnext.intl", "esnext.symbol"
    code typescript
  2. Happy Holidays

    Happy Holidays, everyone. I just launched this blog right into the holiday season, and so start with a gap! I hope your holidays are going well.

    meta
  3. Database vocabulary: ACID

    Databases are often described in terms of their ACID properties (e.g. "eventual consistency"), so knowing the vocabulary is useful when considering a database solution. Some contemporary database management systems intentionally disregard ACID properties in a tradeoff for other gains.

    ACID is an acronym representing four traits of a database transaction that will guarantee valid data even if the transaction is not completed. Transactions that violate these principles will leave the database in an unknown or invalid state.

    • Atomicity: a transaction must either complete all operations of the transaction or no operations.
    • Consistency: a transaction that completes must conform to all and any constraints on the data.
      • eventual consistency: in a distributed database, a change to the data in one instance will take time to propogate to other instances. Eventual consistency means that this change will eventually propogate ("converge"), and all relational constraints and rules will be applied; but before then, anything goes: old, invalid date might be retrieved instead.
    • Isolation: a transaction's operations must not influence (be visible to) other transactions until the transaction is complete. Rather, transactions executed at the same time must behave as if they were executed one at a time.
    • Durability: the changes made by a completed transaction must be permanent.

    The influential paper describing these properties is "Principles of Transaction-Oriented Database Recovery" by Haerder and Reuter, 1983.

    database glossary tech

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