proj4 throws unknown projection: EPSG:4547

• Console throws Error: unknown projection: EPSG:4547 while the same call with EPSG:3857 or EPSG:4326 succeeds.
• EPSG:4526 and EPSG:4490 fail the same way; all affected codes are CGCS2000.
• proj4.defs('EPSG:4547') returns undefined, while proj4.defs('EPSG:3857') returns an object.

1. Run proj4.defs('EPSG:4547') in the console; undefined means it is not registered.
2. Run proj4.defs('EPSG:3857') as a control to confirm the built-in table is available.
3. Look up the PROJ string for the code on epsg.org or in proj4js defs.js.
4. Check the argument order in proj4.defs; the name must come first, as 'EPSG:4547'.
5. After registering, project a point on the central meridian and check that x is exactly 500000.

Register the definition before use: proj4.defs('EPSG:4547', '+proj=tmerc +lat_0=0 +lon_0=111 +k=1 +x_0=500000 +y_0=0 +ellps=GRS80 +units=m +no_defs'); in OpenLayers, call ol.proj.proj4.register(proj4) instead of proj4 directly.

Uncaught Error: unknown projection: EPSG:4547
    at proj4 (proj4.js:1:1)
    at <anonymous>:1:1

console.log(proj4.defs('EPSG:4547'));  // undefined
console.log(proj4.defs('EPSG:3857'));  // { title: 'WGS 84 / Pseudo-Mercator', ... }

• Writing proj4.defs(projString, 'EPSG:4547') with the arguments reversed, which registers nothing useful.
• Using the 4326 definition as a stand-in for 4490; it lacks ellps=GRS80 and hides the datum difference.